Ever-increasing numbers of telephone customers may be coupled to local loop generation equipment, examples of which are security systems, doorbell answering devices, and access control mechanisms. In particular, doorbell answering systems are commonly utilized in multi-family housing units. These systems generally place a switching mechanism in series with the tip/ring lines running from the telephone company central switching office to the customer premises. Normally, this switching mechanism is closed, completing a circuit between the switching office and the customer premises. In this closed state, customers are able to communicate voice and/or data over their telephone lines as if the access control system was not even present. However, this communication is subject to interruption at any time. When a visitor wishes to notify a person at a selected customer premises of his or her presence, the visitor pushes a button, or presses one or more keys on a keypad. The access control system responds to the button or key presses by opening up the circuit between the customer and the central switching office, and by providing a local loop between the visitor and the selected customer premises.
This open circuit is something of a nuisance if it interrupts a voice call already in progress. However, the open circuit is also problematic in cases where the transfer of data is interrupted. For example, many people use a computer modem to access the Internet over conventional telephone lines. Once the circuit between the central office (CO) and the computer modem is broken, the modem will disconnect from the telephone line. The subscriber loses data during this interruption, and may also be faced with the inconvenience of having to re-log into an online service.
The circuit between the central office and the customer is broken so that a local loop may be provided between the customer premises and an access door. After the circuit has been broken, the door answering system then feeds a ringing signal to the telephone line serving the subscriber's premises. When a person at the customer premises takes a telephone off-hook, voice communications are now enabled between this person and the visitor. If this person wishes to grant the visitor access, this person presses a specified DTMF tone sequence on the telephone keypad or, alternatively, presses a lock release button separate and apart from the telephone system to grant the visitor access.
Although the foregoing example deals with local loop generation equipment in the form of a doorbell answering system, other types of local loop generation equipment present similar problems. Whenever the local loop generation equipment creates a local loop, voice and data communications between the customer and the central office are interrupted.
Refer to FIG. 1, which is a hardware block diagram of an illustrative prior art access control system. This access control system places intercom equipment 105 in series between central office 101 and housing unit 103. For the sake of clarity, FIG. 1 shows only one housing unit 103, whereas, in a more typical application, intercom equipment 105 would be placed in series between tip/ring wire pairs running from the central office 101 to each of a plurality of housing units. Tip/ring lines 115, 117 from central office 101 are coupled to a switching mechanism 107 in intercom equipment 105. Switching mechanism 107 selectively switches tip/ring lines 115, 117 to tip/ring lines 119, 121 serving housing unit 103. Normally, switching mechanism 107 is closed, completing a circuit between the switching office and the customer premises by coupling tip line 115 to tip line 119, and ring line 117 to ring line 121. In this manner, customers are now able to place outgoing calls, and also to receive incoming calls, as if the intercom equipment 105 was not even present.
When a visitor wishes to notify a person at a selected housing unit 103 of his or her presence, the visitor pushes a button, or presses one or more keys on a keypad at a lobby interface device 111. In response to the receipt of these keypress signals at switching mechanism 107, microprocessor 109 activates switching mechanism 107 to break the connection between central office 101 and housing unit 103, and to connect housing unit 103 to lobby interface device 111, thereby forming a local loop between lobby interface device 111 and housing unit 103. More specifically, switching mechanism 107 opens up the circuit between tip line 115 and tip line 119, and also between ring line 117 and ring line 121, and closes the circuit between tip line 119 and tip line 123, as well as ring line 121 and ring line 125. The keypress signals are sent out over tip/ring lines 123, 125 which form the local loop between the lobby interface device 111 and the switching mechanism 107. The keypress signals could, but need not, be DTMF signals or pulse dialing signals.
The switching mechanism 107 forwards these keypress signals to microprocessor 109, which responds to the button or key presses by activating switching mechanism 107. The intercom equipment 105 then feeds a ringing signal, via switching mechanism 107, to a telephone at housing unit 103. When a person at housing unit 103 takes the telephone off-hook, voice communications are now enabled between this person and the visitor. If this person wishes to grant the visitor access, this person presses a specified DTMF tone sequence on the telephone keypad or, alternatively, presses a lock release button separate and apart from the telephone system to grant the visitor access.
Unfortunately, whenever a visitor activates the lobby interface device 111 to signal a resident, the resident may already be in data and/or voice communication with central office 101. If, for example, the resident is communicating over the Internet, the Internet connection will typically be lost. These breaks in communication may occur unexpectedly, unpredictably, and repeatedly, causing the resident to become frustrated with the overall quality of telephone service.